The present invention relates to generally a high-pressure sodium lamp of the type whose transparent or translucent arc tube contains a metal such as mercury, cadmium or the like, which vaporizes to form the buffer gas, and sodium, and more particularly the electrodes of the arc tube.
In general, the electrodes of the high-pressure sodium lamps comprise an electrode core and an electrode coil wound around the electrode core in such a way that the electrode core is extended beyond the innermost electrode coil by a predetermined length. One end of the electrode coil, one end of a body of electron-emitting materials or one end of an inner coil carrying the electron-emitting materials (the body of electron-emitting materials and the inner coil are referred to as "the electron-emitting means" hereinafter in this specification) is exposed to the discharge space in which an arc is established, so that at each electrode, the arc spot; that is, the point of contact between the arc and the electrode fluctuates between the end face of the electrode core and the cylindrical surface thereof, one end of the electrode coil or especially one end of the electron-emitting means. As a result, sputtering of the electrode coil and evaporation of the electron-emitting means are accelerated to a considerably higher degree so that tube blackening is accelerated accordingly. In addition, even when a constant voltage is supplied through a stabilizer or ballast, the arc length varies, resulting in the variations in lamp voltage and electrical characteristics of the lamp. Furthermore, the arc spot fluctuation causes variations in operating temperature of the electrode which in turn cause temperature variations the coldest spot in the arc tube. As a consequence, variations in vapor pressure in the arc tube follow in the high-pressure sodium lamps of the saturated vapor type so that lamp voltage variations occur during the lamp life. As a consequence, the electrical as well as optical characteristics vary, so that the factors which influence the lamp life are adversely affected and consequently the lamp life is considerably shortened.
The above-described variations in lamp characteristics due to the arc spot fluctuations are especially pronounced in high-pressure sodium lamps with high-color-rendition in which the average potential gradient is higher than 20 V/cm.
One of the objects of the present invention is, therefore, to provide a high-pressure sodium lamp in which the arc spot fluctuations can be substantially suppressed during operation so that the electrical and optical characteristics of the lamp can be stabilized and the lamp life can be increased.